Wavelength division multiplexing (WDM) technology provides a method of accommodating different wavelength channels in a same fiber.
The channel taking optical signal through the network is referred to wavelength channel. Recently, advanced optical transmission technology has been developed to support high-speed transmission up to 100 Gbps (Giga bit per second) for a single wavelength channel.
As known so far, optical signal degradation can be caused by physical impairments in optical layer, such as, amplifier spontaneous emission (ASE), chromatic dispersion (CD), polarization mode dispersion (PMD) and nonlinearities, etc. The physical impairments mentioned above eventually results in bit errors at receivers of the wavelength channels. With an increase of transmission speed per wavelength channel, optical signals become more vulnerable to physical impairments. In order to realize high reliability in WDM networks, protection mechanism is desired to prevent the optical signals from unintended errors.
Fault management provides such protection mechanism for WDM networks in non patent literature 1. In general, the protection mechanism includes four procedures: failure detection, failure notification, failure localization, and failure recovery.
In failure detection, failures can be identified as signal failure (SF) or signal degradation (SD) by the deployment of optical power level monitors or optical performance monitors. In particular, optical performance monitors can provide monitoring results presented by optical signal noise ratio (OSNR), CD or PMD etc., which can be used to monitor physical impairments. Failure notification may come after the failure detection, which is used to inform the deciding entities to a failure caused by SF or SD. The deciding entities can be the ones making failure recovery decisions. The failure notification can be provided by the optical layer or a control plane. A control plane is a plane or a network, which is used to perform wavelength path configuration. In non patent literature 1, a notifying mechanism is realized by using a generalized multi-protocol label switching (GMPLS)-based control plane. Failure localization provides the information to the deciding entities about the place at which failure occurs. The place may be a node or a link between nodes.
Based on this information, the deciding entity can then make an accurate decision to reroute the traffic to avoid passing the nodes or the links at which failure occurs. On receiving the failure notification, the deciding entity will perform failure recovery to reroute the affected traffic.
A proposal for localizing failure caused by signal degradation is disclosed in Patent Literature 1. In patent literature 1, optical performance monitors are placed at all optical switching equipments (e.g., re-configurable optical add/drop multiplexers (ROADM), wavelength cross-connects (WXC)) or along the fiber link for monitoring all wavelength channels traversing a fiber link. Based on the monitor location information, location of the failure caused by signal degradation can be figured out.
For other related technology, patent literature 2 and patent literature 3 are described as follows.
Patent literature 2 discloses a method of failure detection of a path through which a testing signal passed, by loop-backing the testing signal at nodes on the paths.
Patent literature 3 discloses a method for computing wavelength path group, which would be influenced when signal degradation occurs, based on monitoring results obtained by wavelength dispersion monitors, which are set arbitrarily in a network, and wavelength path information is kept by a network management system (NMS).